The present invention relates in general to an equalization network for use in a repeater stage in a telephone communications system, and in particular, to an equalizer including a reverse equalization network with frequency equalization and a pre-determined frequency cut-off beyond the end of the voice frequency range of telephone communication.
Every wire transmission line is comprised of distributed series resistance, distributed series inductance and shunt capacitance. The frequency response is attenuated for high frequencies to a greater degree than for low frequencies. The degree of such attenuation is a function of line length with the slope of the frequency response increasing with length. To compensate for this phenomenon, a number of slope equalization techniques are known in the prior art. The slope equalizer circuit attempts to compensate for the transmission line characteristics whereby the resultant frequency response is essentially flat over the frequency range of interest.
A "non-loaded" telephone line has the characteristics described above. A "loaded" line has a flatter frequency response because inductors are physically inserted in the line and so as to appropriately load the line. This results in a flatter frequency response in the higher frequency range.
Typical telephone communication systems use two-wire and four-wire lines. The frequency band for these lines is 0.3 KHz to 3.5 KHz for voice communication. Because of frequency related losses in the line, it is necessary to bring the frequency response back up to an essentially flat response in order to restore the signal to the original level and to improve the quality of the transmission. A repeater provides amplification and frequency equalization.
Because of the existence of "non-loaded" lines and "loaded" lines and combinations thereof, and two-wire and four-wire lines, there exists a need for a universal equalizer network. Of particular need is a passive negative slope section in the network, as disclosed by the invention of the applicant, which offsets the frequency characteristics of other sections thereby enabling equalization over the full frequency range of interest. The use of a negative slope section cascaded with other circuit sections to implement a universal equalizer is not found in the prior art.
U.S. Pat. No. 4,012,704 discloses an equalizer comprised of a number of operational amplifier sections wherein passive components are selectively switched into the operational amplifier circuitry to effect the conventional control over circuit Q's, bandwidths, and resonant frequencies. Additionally, the patent teaches the use of a number of variable resistors as part of the input circuitry. These variable resistors do not have a straight negative slope effect, but interact with the active operational amplifier circuitry.
U.S. Pat. No. 3,940,709 discloses an operational amplifier implemented equalization circuit for use at audio frequencies which also teaches the selective insertion by switching of passive networks to provide the desired gain slopes and frequency cut in points. While the disclosure begins to suggest the concept of a "falling in amplitude" capability, the remainder of the disclosure teaches away from this and describes only positive slope results.
The relevance of the prior art indicated in the present specification, should not be given a limited interpretation. A cited prior art item may be found to have relevance at a passage other than one referred to, or to have relevance in the sense different than as stated.